A police officer and a terrorist are among four dead following a major terrorist incident just yards from the UK Houses of Parliament. Here's everything we know about the attack so far.
What happened in Westminster?
Around 2:40pm, an attacker mowed down about a dozen pedestrians on London's Westminster Bridge in a grey Hyundai.
The vehicle then crashed into railings just yards from the Houses of Parliament and the attacker was seen running through the gates of the Palace of Westminster with a knife. He reportedly stabbed a policeman before being shot by armed officers.
Paramedics fought to save his life and that of his victim on the ground of the cobbled courtyard just yards from the House of Commons. Both the policeman are the attacker were later confirmed dead.
Two large knives could be seen at the scene.
Foreign Office minister Tobias Ellwood was among those who rushed to help. Ellwood, who lost his brother in the Bali bombing, could be seen pumping the officer's chest then standing above him, his hands and face smeared with blood.
Scotland Yard has declared the attack a terrorist incident and a counter-terrorism investigation is under way.
Who was the attacker?
We don't know yet, but police believe he was acting alone.
Eyewitnesses described seeing an Asian man in his 40s carrying a knife.
Who are the dead?
Five people are confirmed dead, including the attacker and the police officer who was stabbed by the attacker, Scotland Yard's top anti-terror officer Mark Rowley confirmed. The police officer has been named as Keith Palmer, 48, a husband and father who had 15 years of service.
The other three victims died after the car rampage on Westminster Bridge, including a female pedestrian who became trapped under the wheels of a bus after seemingly being flung there by the attacker's car, according to a junior doctor.
Details of the fifth victim are currently unknown.
What about those injured?
At least 40 people have been seriously injured, including three police officers on their way back from a commendation ceremony and three French teenagers on a school visit to London.
London Ambulance Service said 10 people were treated at Westminster Bridge. Some people are believed to have "catastrophic"injuries.
Two of the French teenagers are believed to be in a "critical condition"while the third is in the operating theatre, according to parents who spoke to local media in Brittany.
The pupils, aged between 15 and 16, are believed to be from a school in Concarneau, Brittany, and had arrived in London on Sunday evening.
They were walking along the bridge when the attacker's car apparently hit them, propelling one of them up onto the bonnet of the vehicle.
Another woman who apparently ended up in the Thames as the attacker drove across the bridge was rescued and given urgent medical treatment on a nearby pier.
Where was the UK Prime Minister?
When the attack happened Prime Minister Theresa May had just voted and was standing in members' lobby with other Tory MPs.
She was seen being quickly ushered into a silver Jaguar in the grounds of the Palace of Westminster. A Downing Street source confirmed she was "OK".
May chaired a meeting of the Government's emergency Cobra committee at Downing Street.
She later said that any attempt to defeat the values that Parliament stands for was "doomed to failure". And she vowed:
"We will all move forward together, never giving in to terror and never allowing the voices of hate and evil to drive us apart."
The UK's terror threat level would remain at "severe", May said.
She praised the bravery of the police officers "who keep us and our democratic institutions safe".
Our thoughts and prayers go out to all who have been affected, to the victims themselves and to their families and friends, who waved their loved ones off but will not now be welcoming them home.
For those of us who were in Parliament at the time of this attack, these events provide a particular reminder of the exceptional bravery of our police and security services who risk their lives to keep us safe.
Once again today, these exceptional men and women ran towards the danger, even as they encouraged others to move the other way.
A No 10 spokesman said: "The thoughts of the PM and the Government are with those killed and injured in this appalling incident, and with their families. The PM is being kept updated and will shortly chair Cobra."
Is London safe?
The area around Westminster is currently on lock down and the public are being urged to avoid the area and remain vigilant. Extra armed and unarmed police are being deployed across the capital.
Facebook has activated its Safety Check feature. People in the area can use the tool to mark themselves as "safe"which will be visible to their friends on the network, as well as to see if friends nearby have done the same.
Londoners have taken to social media to tell those responsible for the terror attack that today's events did not succeed in crushing their spirits and making them afraid.
A defiant trend popped up on Twitter, as people commuting home and going about their business in the Capital in the wake of the attack tweeted "we are not afraid".
How has the world reacted?
Donald Trump has offered the "full co-operation and support"of the United States. The White House said the US president offered his condolences to Theresa May in a phone call following the bloody events in London.
A White House spokesman said: "President Donald J. Trump spoke today with Prime Minister Theresa May of the United Kingdom to offer his condolences on today's terror attack in London and his praise for the effective response of security forces and first responders.
"He pledged the full co-operation and support of the United States government in responding to the attack and bringing those responsible to justice."
President Francois Hollande of France told May in a phone call that a plane from the "French government fleet"would take the families of three injured French high school children to London on Wednesday night. He also conveyed his condolences.
Canadian Prime Minister Justin Trudeau said: "Our thoughts are with the victims of today's attack in London and their families. Canadians remain united with the people of the UK."
Gibraltar's Chief Minister Fabian Picardo said: "Shocked at attack on London. Yesterday I spent the day at Westminster. Luckily not today. London's Parliament is the mother of democracy!"